The present invention relates to a radio transmission system usable mainly in the mobile telephone art. Other uses are feasible, however. The object of the invention is to reduce the cost of equipment by simplifying its design.
Having radio transmit and receive circuits use a common set of local oscillators is known in the art, in particular in the mobile telephone art. Systems using two or three local oscillators are also known in the art. For example, the phase of a signal at an intermediate frequency of the order of 100 MHz produced by a first local oscillator is compared to the phase of a signal produced by a transition local oscillator. The frequency of either or both signals is divided before their phases are compared.
The signal from the phase comparator controls a voltage-controlled third oscillator referred to as the transmission oscillator. Although one of the intermediate frequency and transition frequency local oscillators preferably produces fixed frequency signals and the other produces variable frequency signals, the voltage-controlled local oscillator produces a signal whose frequency varies as a function of the comparison signal. This frequency variation is exploited in the mobile telephone art to provide frequency agility, which changes the frequency of a transmission channel from one transmit or receive frame to the next. To give a more concrete idea of this, the GSM 900 MHz system and the DCS 1800 MHz system have a channel width of 200 kHz.
Frequency agility is achieved by dividing the frequency of the loop signals and of the intermediate frequency signals.
A standard system of the above kind has a number of particular features. Firstly, the need for dual-band mobile telephones, i.e. mobile telephones able to operate in the GSM 900 MHz band and in the DCS 1800 MHz band, can make it necessary to double the number of local oscillators. Accordingly, if no precautions were taken, this could lead to systems with six local oscillators. The cost of such equipment would be excessive. Also, for stable control, the principle of controlling the third local oscillator by means of a signal from a phase comparator requires a loop filter which does not diverge. Although the loop includes a filter of this kind, naturally disposed between the phase comparator and the voltage-controlled oscillator, it also has distributed filtering characteristics which give it a low-pass transfer function with a cut-off frequency slightly different from the cut-off frequency of the filter, although of the same order of magnitude.
The loop cut-off frequency is an important parameter. As it is reduced, the loop becomes more stable but is less able to change state quickly for the frequency hops that provide frequency agility. Furthermore, if there is a requirement to modulate the signal before it enters the loop, this is possible only if the bandwidth of the filter is not too narrow, so that it can pass GMSK modulation. A compromise is therefore required. Furthermore, the presence in the loop of a mixer combining signals from the third local oscillator with signals from the second local oscillator produces mixing noise which propagates in the loop, and as a result the signal from the third local oscillator is noisy. Noise at frequencies close to the carrier frequency is filtered out by the loop, but harmonics are not. Also, the filter damps harmonics of the comparison frequency of the phase comparator. A compromise is therefore required in respect of the filter. This compromise is equivalent to increasing the loop bandwidth to change state faster and decreasing it to eliminate the noise components referred to above.
Of course, a transmitter of the above kind does not merely transmit a carrier signal. It is intended to transmit a carrier signal which has been modulated by a modulation signal. Various solutions to the problem of transmitting the modulation signal in the form of modulation of the transmitted carrier signal can be considered.
A first solution consists of having the modulation signal modulate the intermediate frequency signal before it is fed into the phase comparator. The drawback of that solution is that it requires a large number of local oscillators. The local oscillators cannot be integrated into an integrated circuit including the transmitter system, they complicate its construction, and they increase its overall size. Also, local oscillators are relatively costly.
In another solution, the carrier is modulated in the control loop. In this case, a modulator is placed between the third local oscillator, which produces the transmit frequency, and the second oscillator, which produces the transition frequency. In theory, a modulator of this kind includes a mixer based on a non-linear circuit. Non-linear circuits have the drawback of producing a great deal of noise in addition to modulation. Also, the need to increase the bandwidth of the loop to pass the modulation means that the noise is filtered less and the phase error of the transmitter is increased. The noise is further increased by the presence of the dividers needed for frequency agility.
The higher the loop division coefficients, the more noise is produced. Dividers which divide by 9000 or by 4500 to produce signals at frequencies of the order of 200 kHz, comparable to an intermediate frequency signal, are known in the art.
The invention seeks to solve the noise problem whilst reducing the number of local oscillators needed. These two effects are obtained simultaneously simply by replacing the mixers or dividers in the loop, which have high coefficients to enable reduction of the frequency, with fractional dividers which then enable the comparator to compare a first signal whose frequency has been divided in this way and a second signal at an intermediate frequency, which is preferably a frequency higher than those used in the prior art. In one particularly judicious instance, this leads to the use of a single local oscillator, the intermediate frequency oscillator merely consisting of a very stable 13 MHz clock which is already available in an integrated circuit which includes the transmission system.
Secondly, the solution of the invention does not call for modulation index division, which would require pre-amplification of the modulation signal, since the transmission signal is modulated directly in the control loop before frequency division. This automatically reduces the resolution of the digital part of the system and therefore simplifies the processing circuits and the control logic circuits.
Also, it can be shown that a fractional divider produces only noise that is related to the integer part of the division coefficient and is therefore at a significantly lower level than the noise produced by dividers with a high coefficient, by a ratio of approximately 1 to 30. Accordingly, the presence in the loop of the low-pass filter with a cut-off frequency of approximately 200 kHz is sufficient to eliminate residual noise and to comply with the standards. Accordingly, increasing the loop bandwidth is not detrimental to noise performance (there is no increase in the phase error). With the synthesizer of the invention, a greater loop bandwidth can be retained, which is beneficial because it procures a higher switching or hopping rate for frequency agility.
The invention therefore provides a radio transmission system including, in a phase control loop, a voltage-controlled oscillator producing a signal to be transmitted, a modulator modulating the signal to be transmitted by a modulation signal to produce a modulated signal, and a phase comparator receiving on a phase input a signal representative of the modulated signal and on a second input a reference signal at an intermediate frequency, wherein the control loop includes a fractional frequency divider between the modulator and the comparator.